1. Field of the Invention
The present invention relates to a method of chromatic dispersion compensation which includes the function of pulse waveform shaping, and particularly to an adaptive dispersion compensation device and a control method for the device which are useful for the super-high speed optical fiber communication.
2. Description of the Prior Art
The optical fiber communication which has innovated on the trunk system is advancing toward the subscriber system in recent years.
In practicing the long-distance or high-speed transmission of optical signals having wavelengths of 1.5-xcexcm band through a 1.3-xcexcm band zero-dispersion optical fiber cable which is most prevalent currently, it is necessary for the prevention of decay of optical signals to provide a means of dispersion control against the chromatic dispersion of around 17 ps/kmxc2x7nm that is inherent in optical fiber.
The conventional dispersion control means is typically a dispersion compensator which is based on a chirp-bragg fiber grating having a structure of continuously varied reflectivity modulation periods. The chirp-bragg fiber grating which forms a diffraction grating within the core of optical fiber cable has a property of light reflection of a specific wavelength. By forming a diffraction grating (chirp-bragg fiber grating) which varies the pitch continuously along the axis of optical fiber, it becomes a device of having a reflection position which depends on the wavelength of light.
By utilizing this property of chirp-bragg fiber grating, a dispersion compensator is built. The chirp-bragg fiber grating, when combined with an optical circulator, is equivalent in function to a dispersion compensation fiber, while being compact. While the conventional chirp-bragg fiber gratings have static characteristics of dispersion and reflection in most cases, it is desirable for many applications such as dispersion compensation to have diffraction gratings which are controllable in terms of band or dispersion.
A prior art example which attempted to introduce a dynamically adjustable chirp to a chirp-bragg fiber grating appeared as xe2x80x9coptical diffraction grating device having adjustable chirpxe2x80x9d in Japanese Patent Unexamined Publication No.2000-137197.
FIG. 1 shows the fabrication process of this chirp diffraction grating, which includes: A. preparation of an optical waveguide inclusive of a diffraction grating, B. coating of the diffraction grating region with a variable resistance thin film, and C. packaging of device.
The first step A prepares an optical waveguide of a certain length which includes a diffraction grating. The waveguide is optical fiber which is not coated preferably or may include an electrically insulated resistance thin film of uniform resistance. The waveguide can be of either single mode or multi-mode. The diffraction grating can be either a bragg diffraction grating or a long-period diffraction grating. The second step B coats the waveguide with such a thin film of resistance material that the local resistance increases continuously along the axis of diffraction grating. The third step C, which is implemented only when necessary, encases the device in a package.
FIG. 2 is a brief cross-sectional diagram, showing a specific example of the structure of waveguide diffraction grating device having an adjustable chirp. Indicated by 10 is optical fiber, 11 is a diffraction grating, 12 is the perturbation of refraction, 13 is a base body, and 14 and 15 are electrodes.
The optical waveguide diffraction grating of adjustable chirp includes a waveguide diffraction grating which is thermally in contact with a thermal conversion base body which varies in temperature along the axis of diffraction grating and is controllable electrically. The thermal conversion base body, which creates a temperature gradient along the diffraction grating, is capable of generating or absorbing heat on the fiber. In one example, the thermal conversion base body is a resistance coat film which varies in local resistance along the axis of diffraction grating. A current flowing in the thin film produces a temperature gradient which is virtually proportional to the local resistance of thin film, and the magnitude of chirp can be adjusted based on the value of current. This device is simple, compact and high in power efficiency.
A femto-second optical scope which is a means of detecting the nature of a signal light in optical fiber transmission is described in an article entitled xe2x80x9cHigh-resistivity pulse spectrogram measurement using two-photon absorption in a semiconductor at 1.5-xcexcm wavelengthxe2x80x9d in publication OPTICS EXPRESS, Vol.7, No.3 (published on Jul. 31, 2000), pp.135-140. The femto-second optical scope is designed inherently to make a two-dimensional map between the wavelength and the delay time based on the measurement of the time dependency and wavelength dependency of the phase of a super-high speed light pulse of the order of femto seconds, and it is capable of computing the second, third and fourth-order dispersion values of the optical transmission line based on the implementation of curve fitting.
However, in regard to the above-mentioned means with the chirp-bragg fiber grating, the manner of light detection for the compensation of dispersion in response to the variation of transmission state and transmission distance and the manner of feedback control of the dispersion characteristics based on the light detection are not described, and this means is problematic in that it cannot deal flexibly with the actual super-high speed light pulse transmission. Furthermore, the resistance member, which is designed to create a temperature gradient based on the control of the amount of heat generation by varying the resistance value in accordance with the progressive variation of thin film thickness, is problematic in that it is difficult to control the high-order (third order and above) dispersion which is higher than the chromatic dispersion (second order dispersion).
With the intention of solving the foregoing prior art problems, it is an object of the present invention to provide an adaptive dispersion compensation device which performs the dispersion compensation inclusive of waveform shaping in adaptive fashion for the optical fiber transmission.
Another object of the present invention is to provide a control method for the adaptive dispersion compensation device.
In order to achieve the above objectives, the present invention resides in an adaptive dispersion compensation device which is arranged to include a plurality of basic units each including a chirp-bragg fiber grating which is formed in an optical waveguide, a reflection mirror which is disposed on the light input side of the chirp-bragg fiber grating by being detachable and an optical circulator which is connected to the reflection mirror, means of connecting the basic units in series, and means of controlling the dispersion characteristics of each chirp-bragg fiber grating by applying a temperature gradient to it along its axis.
In case there is no reflection mirror placed in the basic unit, the optical circulator is connected to the chirp-bragg fiber grating by an optical fiber with optical connectors so that the input light is put directly into the grating. In case a reflection mirror is placed in the basic unit, the entire input light can be reflected by it or part of the input light can be conducted by it to reach the chirp-bragg fiber grating, with the rest being reflected, depending on its reflectivity.
The basic units have individual dispersion characteristics combined to make the dispersion characteristics of the whole device, thereby performing the dispersion compensation or waveform shaping in adaptive fashion for the optical fiber transmission.
The inventive adaptive dispersion compensation device is also arranged to include a femto-second optical scope which detects the nature of the output signal light and means of controlling the dispersion characteristics of each basic unit in feedback fashion based on the dispersion value computed from the detected nature of the output signal light. By the detection of a super-high speed light pulse of the order of femto seconds and the dispersion control based on the pulse detection, adaptive control can readily be performed.
The inventive adaptive dispersion compensation device is also arranged to include means of controlling the dispersion characteristics of each chirp-bragg fiber grating by applying a tension to it along its axis.
The inventive adaptive dispersion compensation device is also arranged to include means of controlling the dispersion characteristics of each basic unit in feedback fashion based on the dispersion value of the third or higher order computed from the nature of the output signal light.
The present invention also resides in a method of controlling an adaptive dispersion compensation device which includes the operational steps of entering a control signal for setting a temperature gradient of each chirp-bragg fiber grating to a dispersion characteristics control means, applying the temperature gradient, which is produced by the dispersion characteristics control means in accordance with the control signal, to each chirp-bragg fiber grating along its axis, and controlling the dispersion quality of the input light with the chirp-bragg fiber gratings having the application of the temperature gradients.
As described above, the inventive adaptive dispersion compensation device is capable of performing the dispersion compensation inclusive of waveform shaping in adaptive fashion for the optical fiber transmission, and it is compact and stable in operation. The inventive adaptive dispersion compensation device and its control method are capable of performing the dispersion compensation inclusive of waveform shaping in adaptive fashion for the optical fiber transmission while monitoring the light signal on the optical fiber transmission line.
These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.